selftests/nolibc: Add 7 tests for memcmp()

This adds 7 combinations of input values for memcmp() using signed and
unsigned bytes, which will trigger on the original code before Rasmus'
fix. This is mostly aimed at helping backporters verify their work, and
showing how tests for corner cases can be added to the selftests suite.

Before the fix it reports:
  12 memcmp_20_20 = 0                      [OK]
  13 memcmp_20_60 = -64                    [OK]
  14 memcmp_60_20 = 64                     [OK]
  15 memcmp_20_e0 = 64                    [FAIL]
  16 memcmp_e0_20 = -64                   [FAIL]
  17 memcmp_80_e0 = -96                    [OK]
  18 memcmp_e0_80 = 96                     [OK]

And after:
  12 memcmp_20_20 = 0                      [OK]
  13 memcmp_20_60 = -64                    [OK]
  14 memcmp_60_20 = 64                     [OK]
  15 memcmp_20_e0 = -192                   [OK]
  16 memcmp_e0_20 = 192                    [OK]
  17 memcmp_80_e0 = -96                    [OK]
  18 memcmp_e0_80 = 96                     [OK]

Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>