Add POSIX swab.

Surprisingly to me, there are actual uses of `swab` in the codebases
I have available to search, including one with a #ifndef __ANDROID__
around it.

Bug: N/A
Test: ran tests
Change-Id: Ic91b78ae22bb65c346cb46dd38916f48d979abe0
diff --git a/libc/libc.arm64.map b/libc/libc.arm64.map
index 29c5235..2ba32bf 100644
--- a/libc/libc.arm64.map
+++ b/libc/libc.arm64.map
@@ -1286,6 +1286,7 @@
     sethostent;
     setnetent;
     setprotoent;
+    swab;
     syncfs;
 } LIBC_O;