Restore Solaris build after introducing 'noreturn' (re: c4f980eb)

The build started failing on Solaris Studio cc when 'noreturn' was
introduced, because the wrappers pass the -xc99 flag which sets the
compiler to C99 mode. 'noreturn' is a C11 feature. The
stdnoreturn.h header was correctly included but the compiler still
threw a syntax error (long path abbreviated below):

".../stk.c", line 124: warning: _Noreturn is a keyword in ISO C11
".../stk.c", line 124: warning: old-style declaration or incorrect
						type for: _Noreturn
".../stk.c", line 124: syntax error before or at: static

src/cmd/INIT/cc.sol11.*:
- Pass -std=c11 to cc instead of -xc99. At least on i386-64, this
  is sufficient to fix the build.

README.md, src/cmd/ksh93/README.md:
- Remove -xc99 from the Solaris build flags example as that is
  incompatible with -std=c11 (and was already redundant with the
  -xc99 in the wrappers).

src/cmd/ksh93/tests/basic.sh:
- Don't run a newly backported 93v- regression test on Solaris
  because it uses the 'join' command with process subsitutions;
  Solaris 11.4's join(1) hangs when trying to read from /dev/fd.
  This is not ksh's fault. (re: 59bacfd4)

src/cmd/INIT/cc.sol11.i386

@@ -1,4 +1,4 @@
-: solaris.i386 cc wrapper for reasonable ansi C defaults and 32 bit : 2021-01-17 :
+: solaris.i386 cc wrapper for 32 bit : 2021-05-12 :
 
 HOSTTYPE=sol11.i386
 
@@ -15,4 +15,4 @@ esac
 
 # Note: the _XPG6 macro is now defined in src/lib/libast/features/common
 
-$CC_EXPLICIT -m32 -xc99 "$@"
+$CC_EXPLICIT -m32 -std=c11 "$@"

src/cmd/INIT/cc.sol11.i386-64

@@ -1,4 +1,4 @@
-: solaris.i386-64 cc wrapper for reasonable ansi C defaults and 64 bit : 2021-01-17 :
+: solaris.i386-64 cc wrapper for 64 bit : 2021-05-12 :
 
 HOSTTYPE=sol11.i386-64
 
@@ -15,4 +15,4 @@ esac
 
 # Note: the _XPG6 macro is now defined in src/lib/libast/features/common
 
-$CC_EXPLICIT -m64 -xc99 "$@"
+$CC_EXPLICIT -m64 -std=c11 "$@"

src/cmd/INIT/cc.sol11.sparc

@@ -1,4 +1,4 @@
-: solaris.sparc cc wrapper for reasonable ansi C defaults and 32 bit : 2021-01-17 :
+: solaris.sparc cc wrapper for 32 bit : 2021-05-12 :
 
 HOSTTYPE=sol11.sparc
 
@@ -15,4 +15,4 @@ esac
 
 # Note: the _XPG6 macro is now defined in src/lib/libast/features/common
 
-$CC_EXPLICIT -m32 -xc99 "$@"
+$CC_EXPLICIT -m32 -std=c11 "$@"

src/cmd/INIT/cc.sol11.sparc-64

@@ -1,4 +1,4 @@
-: solaris.sparc-64 cc wrapper for reasonable ansi C defaults and 64 bit : 2021-01-17 :
+: solaris.sparc-64 cc wrapper for 64 bit : 2021-05-12 :
 
 HOSTTYPE=sol11.sparc-64
 
@@ -15,4 +15,4 @@ esac
 
 # Note: the _XPG6 macro is now defined in src/lib/libast/features/common
 
-$CC_EXPLICIT -m64 -xc99 "$@"
+$CC_EXPLICIT -m64 -std=c11 "$@"