tools/nolibc: add ftruncate()

tools/nolibc: add ftruncate()

On architectures with 32-bit longs, call the compat syscall
__NR_ftruncate64. As off_t is 64-bit it must be split into 2 registers.
Unlike llseek() which passes the high and low parts in explicitly named
arguments, the order here is endian independent.

Some architectures (arm, mips, ppc) require this pair of registers to
be aligned to an even register, so add custom _sys_ftruncate64()
wrappers for those.

A test case for ftruncate is added which validates negative length or
invalid fd return the appropriate error, and checks the length is
correct on success.

Co-developed-by: Jordan Richards <jordanrichards@google.com>
Signed-off-by: Jordan Richards <jordanrichards@google.com>
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Willy Tarreau <w@1wt.eu>
Reviewed-by: Daniel Palmer <daniel@thingy.jp>
Link: https://patch.msgid.link/20260521-nolibc-ftruncate-v1-3-5384a83b2402@weissschuh.net