riscv: Optimize gp-relative linker relaxation

Changes compiler options and the linker script to group most small data
objects together in a new .small section, and point __global_pointer$ at
.small so that nearly all objects can be referenced via gp.

Signed-off-by: Stefan O'Rear <sorear@fastmail.com>

CMakeLists.txt

@@ -286,7 +286,12 @@ elseif(KernelArchARM)
     endif()
 
 elseif(KernelArchRiscV)
-    KernelCommonFlags(-mcmodel=medany)
+    # Group "small" data objects together in a small-data section so they can
+    # be referenced using gp-relative addressing.  The exact value of
+    # small-data-limit is not crucial but it should be lowered if .small
+    # exceeds 4KiB.
+    #
+    KernelCommonFlags(-mcmodel=medany -msmall-data-limit=1024)
 
 else()
     message(FATAL_ERROR "unknown KernelArch '${KernelArch}'")

src/arch/riscv/common_riscv.lds

@@ -48,12 +48,13 @@ SECTIONS
 
     /* Start of data section */
     _sdata = .;
-    .sdata : {
+    .small : {
+        /* Small data that should be accessed relative to gp.  ld has trouble
+           with the relaxation if they are not in a single section. */
         __global_pointer$ = . + 0x800;
-        *(.sdata*)
-    }
-    .srodata : {
         *(.srodata*)
+        *(.sdata*)
+        *(.sbss)
     }
 
     .rodata . : AT(ADDR(.rodata) - KERNEL_OFFSET)
@@ -71,7 +72,6 @@ SECTIONS
     {
         *(.bss)
         *(COMMON) /* fallback in case '-fno-common' is not used */
-        *(.sbss)
 
         /* 4k breakpoint stack */
         _breakpoint_stack_bottom = .;