Fix ASan stack range registration for main task

[chenBright]

What problem does this PR solve?

Issue Number: resolve

Problem Summary:

Under ASan, every bthread_exit from a non-main task (i.e. the
throw bthread::ExitException(retval) branch) reliably emits the
following warning to stderr, polluting test output and confusing future
ASan reports:

==42417==WARNING: ASan is ignoring requested __asan_handle_no_return: stack type: default top: 0x7feebb8cc000; bottom 0x7feebc8c9000; size: 0xffffffffff003000 (-16764928)
False positive error reports may follow
For details see https://github.com/google/sanitizers/issues/189

(top - bottom ≈ 1.5 GiB is impossible for any real stack: brpc's
biggest bthread stack is STACK_TYPE_LARGE, an order-of-magnitude
smaller, and a default pthread stack is only a few MB.)

Root cause: brpc's StackStorage::bottom is, by convention, the
highest address of the stack (see bthread/stack.h comment "Assume
stack grows upwards"; stack.cpp confirms with
s->bottom = (char*)mem + stacksize). However, in the ASan-only branch
of TaskGroup::init, the value passed to stk->storage.bottom for the
main task came straight from PthreadAttrGetStack, which on Linux uses
pthread_attr_getstack(3) — and that returns the lowest address of
the region. The convention was therefore violated for the main task.

internal::StartSwitchFiber computes
asan_stack_bottom = (char*)storage.bottom - storage.stacksize and
passes it to __sanitizer_start_switch_fiber. With the convention
violated, this address was stacksize bytes below the real pthread
stack, so the per-jump BTHREAD_SCOPED_ASAN_FIBER_SWITCHER told ASan
that the main task lives in nonsensical memory. From that point on,
ASan's per-thread fake-stack tracking was inconsistent with reality.
The first time the C++ unwinder fired (e.g. from
throw bthread::ExitException), __asan_handle_no_return saw a stack
range that straddles the bthread stack and the bogus "main" stack and
bailed out with the warning above.

What is changed and the side effects?

Changed:

PthreadAttrGetStack now translates
the lowest address returned by pthread_attr_getstack(3) into a
highest address before returning, so callers can keep treating the
output as a StackStorage::bottom. macOS already returns the stack
base (highest address) via pthread_get_stackaddr_np(3) and is
unchanged. A comment is added describing the convention.

Before fix:

[ RUN      ] BthreadTest.bthread_exit
I0510 08:26:19.082902 42422 210453414407 bthread_unittest.cpp:125 just_exit] just_exit(0)
I0510 08:26:19.083010 42427 214748387572 bthread_unittest.cpp:125 just_exit] just_exit(0)
I0510 08:26:19.083033 42426 141733932703 bthread_unittest.cpp:125 just_exit] just_exit(0)
==42417==WARNING: ASan is ignoring requested __asan_handle_no_return: stack type: default top: 0x7feebb8cc000; bottom 0x7feebc8c9000; size: 0xffffffffff003000 (-16764928)
False positive error reports may follow
For details see https://github.com/google/sanitizers/issues/189
I0510 08:26:19.083066 42422 176093685563 bthread_unittest.cpp:125 just_exit] just_exit(0)
I0510 08:26:19.083850 42431     0 bthread_unittest.cpp:125 just_exit] just_exit(0)
[       OK ] BthreadTest.bthread_exit (1 ms)

After fix:

[ RUN      ] BthreadTest.bthread_exit
I0513 14:07:54.980260 42664 193273544132 bthread_unittest.cpp:125 just_exit] just_exit(0)
I0513 14:07:54.980327 42668 236223216497 bthread_unittest.cpp:125 just_exit] just_exit(0)
I0513 14:07:54.980370 42670 266287994733 bthread_unittest.cpp:125 just_exit] just_exit(0)
I0513 14:07:54.980438 42664 210453406139 bthread_unittest.cpp:125 just_exit] just_exit(0)
I0513 14:07:54.980901 42671     0 bthread_unittest.cpp:125 just_exit] just_exit(0)
[       OK ] BthreadTest.bthread_exit (1 ms)

Side effects:

• Performance effects:

• Breaking backward compatibility:

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Check List:

• Please make sure your changes are compilable.
• When providing us with a new feature, it is best to add related tests.
• Please follow Contributor Covenant Code of Conduct.