gdb/testsuite: Multiple improvements for gdb.reverse/insn-reverse.exp

gdb/testsuite: Multiple improvements for gdb.reverse/insn-reverse.exp

This commits tackles 2 problems in the test
gdb.reverse/insn-reverse.exp. They are, broadly: flawed logic when an
unexpected error occurs, and badly formed asm expressions.

For the first, what happens is that if the inferior stops progressing
for some reason, the test will emit an UNSUPPORTED and continue testing
by reversing from the current location and checking all registers for
every instruction.  However, due to how the outputs are indexed in the
test, this early exit will cause most of the subsequent tests to be
de-synced and will emit many unrelated failures.

This commit changes the UNSUPPORTED for a FAIL, since the test has in
fact failed to record the execution of the whole function, and
decrements the recorded instruction count by one so that the indexes are
in sync once more.

At the time of committing, this reduces the amount of failures when
testing with clang-15 from around 150 to 2, and correctly identifies
where the issue lies.

The second problem is in how the asm statements in the *-x86.c file
are written. As an example, let's examine the following line:

__asm__ volatile ("rdrand %%ebp;" : "=r" (number));

This statement says that number is being used as the output variable,
but is not indicating which registers were clobbered so that the
compiler is able to properly output. GCC decides to just not save
anything, whereas clang assumes that the output is in %rax, and writes
it to the variable. This hid the problem that any compiler is not good
at dealing with asm statements that change the rbp register. It can be
seen more explicitly by informing gcc that rbp has been clobbered like
so:

__asm__ volatile ("rdrand %%ebp;" : "=r" (number) : : "%ebp");

This statement gets compiled into the following assembly:
        rdrandl %ebp
        movl    %eax, -4(%rbp)

Which is clearly using the incorrect rbp to find the memory location of
the variable. Since the test only exercises GDB's ability to record the
register changes, this commit removes the output to memory.

Finally, correctly informing the compiler of clobbered registers
makes gcc throw an error that the rsp is no longer usable at the end of
the function. To avoid that, this commit compresses the 3 asm statements
that would save, change and reset registers into a single asm statement.

Approved-By: Tom Tromey <tom@tromey.com>