[hwasan] Fix rare false negative (zero tag) in two more test cases (#69491)

stack-uas.c and stack-history-length.c both have
-hwasan-record-stack-history=libcall, which makes the stack base
tag fully randomized. They may therefore sometimes have a zero tag
for a stack allocated variable, resulting in a false negative

(https://github.com/llvm/llvm-project/issues/69221#issuecomment-1767322411).

This patch applies the same workaround as used for deep-recursion.c

(https://github.com/llvm/llvm-project/commit/aa4dfd3736dd1c2e0263eacd09bd613c5784ea73)
and stack-uar.c

(https://github.com/llvm/llvm-project/commit/ddf1de20a3f7db3bca1ef6ba7e6cbb90aac5fd2d):
have two adjacent stack-allocated variables, and use whichever is not
zero-tagged.

These are the last remaining test cases that use
-hwasan-record-stack-history=libcall.

stack-uas flakiness spotted in the wild:
https://lab.llvm.org/buildbot/#/builders/269/builds/549/steps/11/logs/stdio
stack-history-length:
https://lab.llvm.org/buildbot/#/builders/269/builds/537

Co-authored-by: Thurston Dang <thurston@google.com>

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