[AIX] Make errno EWOULDBLOCK an alias of EAGAIN

[xingxue-ibm]

Description

Although the POSIX standard requires errno values to be distinct, it allows EWOULDBLOCK to have the same value as EAGAIN (https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/errno.h.html).

[EWOULDBLOCK] Operation would block (may be the same value as [EAGAIN]). 

Sources

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI

[tgross35]

Thanks!

[xingxue-ibm]

Thank you, @tgross35!

[xingxue-ibm]

@rustbot label +stable-nominated