std: fix path joining on UEFI
std: fix path joining on UEFI

UEFI uses `\` for paths exclusively. This changes std.fs.path to use `\`
for UEFI path joining. Also adds a few tests regarding it, specifically
in making sure double-separators do not result from path joining, as the
UEFI spec says to convert any that result from joining into single
separators (UEFI Spec Version 2.7, pg. 448).

andrewrk/zig,andrewrk/zig