bootstrap: Avoid TOC overflow on AIX powerpc

Since commit 5c58a7e4d2 (ppc64: Work around TOC overflow with platform
specific linker flags, 2019-02-27, v3.15.0-rc1~460^2) we use a bigtoc
flag on this platform when building CMake with CMake.  Add it to the
bootstrap script too.

bootstrap

@@ -157,6 +157,13 @@ else
   cmake_system_hpux=false
 fi
 
+# Determine whether this is AIX
+if echo "${cmake_system}" | grep AIX >/dev/null 2>&1; then
+  cmake_system_aix=true
+else
+  cmake_system_aix=false
+fi
+
 # Determine whether this is Linux
 if echo "${cmake_system}" | grep Linux >/dev/null 2>&1; then
   cmake_system_linux=true
@@ -1109,6 +1116,13 @@ if ${cmake_system_haiku}; then
   cmake_ld_flags="${LDFLAGS} -lroot -lbe"
 fi
 
+# Add AIX arch-specific link flags.
+if ${cmake_system_aix}; then
+  if uname -p | grep powerpc >/dev/null 2>&1; then
+    cmake_ld_flags="${LDFLAGS} -Wl,-bbigtoc"
+  fi
+fi
+
 #-----------------------------------------------------------------------------
 # Detect known toolchains on some platforms.
 cmake_toolchains=''