sanitizer: Fix asan against glibc 2.34 [PR100114]

sanitizer: Fix asan against glibc 2.34 [PR100114]

As mentioned in the PR, SIGSTKSZ is no longer a compile time constant in
glibc 2.34 and later, so
static const uptr kAltStackSize = SIGSTKSZ * 4;
needs dynamic initialization, but is used by a function called indirectly
from .preinit_array and therefore before the variable is constructed.
This results in using 0 size instead and all asan instrumented programs
die with:
==91==ERROR: AddressSanitizer failed to allocate 0x0 (0) bytes of SetAlternateSignalStack (error code: 22)

Here is a cherry-pick from upstream to fix this.

2021-04-17  Jakub Jelinek  <jakub@redhat.com>

	PR sanitizer/100114
	* sanitizer_common/sanitizer_posix_libcdep.cpp: Cherry-pick
	llvm-project revisions 82150606fb11d28813ae6da1101f5bda638165fe
	and b93629dd335ffee2fc4b9b619bf86c3f9e6b0023.