[libc] Template the writing mode for the writer class (#111559)

Summary:
Currently we dispatch the writing mode off of a runtime enum passed in
by the constructor. This causes very unfortunate codegen for the GPU
targets where we get worst-case codegen because of the unused function
pointer for `sprintf`. Instead, this patch moves all of this to a
template so it can be masked out. This results in no dynamic stack and
uses 60 VGPRs instead of 117. It also compiles about 5x as fast.

Author: jhuber6

libc/config/config.json

@@ -41,6 +41,10 @@
     "LIBC_CONF_PRINTF_DISABLE_STRERROR": {
       "value": false,
       "doc": "Disable handling of %m to print strerror in printf and friends."
+    },
+    "LIBC_CONF_PRINTF_RUNTIME_DISPATCH": {
+      "value": true,
+      "doc": "Use dynamic dispatch for the output mechanism to reduce code size."
     }
   },
   "scanf": {

libc/config/gpu/amdgpu/config.json

@@ -19,6 +19,9 @@
     },
     "LIBC_CONF_PRINTF_DISABLE_STRERROR": {
       "value": true
+    },
+    "LIBC_CONF_PRINTF_RUNTIME_DISPATCH": {
+      "value": false
     }
   },
   "scanf": {