libc.map.txt: remove the two riscv64 special cases.

Yes, sigblock() and sigsetmask() are obsolete, but -- despite being the one responsible for this in the first place -- I'm struggling to justify a pointless difference between riscv64 and arm64/x86-64, given that it's _extra_ work to not have the symbols.

Change-Id: Ic58733c3dcfb71df8f92c3670710baac1cbd2680

Author: enh-google

libc/libc.map.txt

@@ -955,7 +955,7 @@ LIBC {
     sigaction;
     sigaddset;
     sigaltstack;
-    sigblock; # arm x86 arm64 x86_64
+    sigblock;
     sigdelset;
     sigemptyset;
     sigfillset;
@@ -968,7 +968,7 @@ LIBC {
     sigprocmask;
     sigqueue; # introduced=23
     sigsetjmp;
-    sigsetmask; # arm x86 arm64 x86_64
+    sigsetmask;
     sigsuspend;
     sigtimedwait; # introduced=23
     sigwait;