[PATCH][ASAN] Switch off by default allocas/VLA sanitization for KASAN

[Maxim Ostapenko]

Hi,

since kernel doesn't support __asan_alloca_poison and 
__asan_allocas_unpoison runtime calls so far, the allocas/VLAs 
sanitization patch 
(https://gcc.gnu.org/ml/gcc-patches/2017-07/msg00258.html) will break 
KASan builds.
So it was decided to introduce an option --param 
asan-instrument-allocas=0/1 (on by default for userspace and off for 
kernel) to avoid the issue.

Tested on x86_64-unknown-linux-gnu, OK after 
https://gcc.gnu.org/ml/gcc-patches/2017-07/msg00258.html will be applied?

-Maxim
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