[build] Don't assume x64 host when setting up gcc toolchains.

Change-Id: Ie79c6e415ef287472c41597f6896ce1045e01fa7
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/304361
Reviewed-by: Alexander Aprelev <aam@google.com>

build/toolchain/linux/BUILD.gn

@@ -163,7 +163,7 @@ gcc_toolchain("clang_x86") {
 }
 
 gcc_toolchain("x86") {
-  prefix = ""
+  prefix = "i686-linux-gnu-"
   if (ia32_toolchain_prefix != "") {
     prefix = ia32_toolchain_prefix
   }
@@ -205,7 +205,7 @@ gcc_toolchain("clang_x64") {
 }
 
 gcc_toolchain("x64") {
-  prefix = ""
+  prefix = "x86_64-linux-gnu-"
   if (x64_toolchain_prefix != "") {
     prefix = x64_toolchain_prefix
   }