[fuzzer] Restrict merge-sigusr.test to Linux (#216702)

The `fuzzer/merge-sigusr.test` test causes the whole `ninja check-all`
run on NetBSD to be killed with `SIGUSR2`.

It turns out the test is highly Linux-specific in at least two ways:

- The `setsid` command doesn't exist on any of Darwin, FreeBSD, and
NetBSD.

- `ps -o sess= <pid>` is highly unportable, too:

  - FreeBSD `ps` doesn't have the `sess` keyword at all.

- NetBSD `ps` does, but with different semantics: it's the session
pointer, not the session id as on Linux, which leads to randomly killing
unrelated processes.

Therefore this patch restricts the test to Linux instead of simply
skipping it on Darwin and Windows.

Tested on `x86_64-pc-netbsd11.0`, `x86_64-pc-linux-gnu`, and
`x86_64-pc-freebsd15.1`.

(cherry picked from commit f2c31b8ffd99378e099720d7ddbf8078c650543a)

compiler-rt/test/fuzzer/merge-sigusr.test

@@ -1,7 +1,7 @@
 # Check that libFuzzer honors SIGUSR1/SIGUSR2
-# FIXME: Disabled on Windows for now because of reliance on posix only features
-# (eg: export, "&", pkill).
-UNSUPPORTED: darwin, target={{.*windows.*}}
+# Requires Linux features (setsid, ps -o sess=).  Also relies on posix only
+# features (eg: export, "&", pkill).
+REQUIRES: linux
 RUN: rm -rf %t
 RUN: mkdir -p %t
 RUN: %cpp_compiler %S/SleepOneSecondTest.cpp -o %t/LFSIGUSR