[GWP-ASan] Add inbuilt options parser.

Adds a modified options parser (shamefully pulled from Scudo, which
shamefully pulled it from sanitizer-common) to GWP-ASan. This allows
customers (Android) to parse options strings in a common way.

Depends on D94117.

AOSP side of these patches is staged at:

 - sepolicy (sysprops should only be settable by the shell, in both root and
 unrooted conditions):
 https://android-review.googlesource.com/c/platform/system/sepolicy/+/1517238

 - zygote updates:
 https://android-review.googlesource.com/c/platform/frameworks/base/+/1515009

 - bionic changes to add `gwp_asan.<process_name>` system property, and
 GWP_ASAN_OPTIONS environment variable:
 https://android-review.googlesource.com/c/platform/bionic/+/1514989

Reviewed By: eugenis

Differential Revision: https://reviews.llvm.org/D92696