Attempt to be better with signed conversions
Attempt to be better with signed conversions

Most targets use unsigned chars.  Explicitly change the static tables to
`signed char`.  An alternative  would be to force signed chars during
the compilation of libdispatch.  When building for Linux AArch64, the
use of `-1` causes signed conversion warnings when initializing the
table.

Signed-off-by: Daniel A. Steffen <bc823475b60dbcbd7be1bbdfe095b5f1939d5bd2@apple.com>

apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch