layout: Use .bss section and only use a single PHDR

Adding a bss section reduces binary size by ~30%. We also only use a
single Program Header. This better reflects the fact that the entire
firmware is loaded as a single Read/Write/Execute blob.

Right now, the hypervisor's loader does not read PHDR types:
    https://github.com/rust-vmm/linux-loader/blob/e5c6d66d3121421672c9b25b02e8954f0ed5f58d/src/loader/mod.rs#L248-L274

layout.ld

@@ -2,9 +2,7 @@ ENTRY(ram64_start)
 
 PHDRS
 {
-  rodata PT_LOAD FILEHDR PHDRS ;
-  data   PT_LOAD               ;
-  text   PT_LOAD               ;
+  program PT_LOAD FILEHDR PHDRS ;
 }
 
 /* Loaders like to put stuff in low memory (< 1M), so we don't use it. */
@@ -19,12 +17,10 @@ SECTIONS
 	. = ram_min;
 	. += SIZEOF_HEADERS;
 
-	.rodata : { *(.rodata .rodata.*)            } :rodata
-	.data   : { *(.data .data.*) *(.bss .bss.*) } :data
-	.text   : {
-		*(.text .text.*)
-		*(.ram64)
-	} :text
+	.rodata : { *(.rodata .rodata.*) } :program
+	.text   : { *(.text .text.*)     } :program
+	.data   : { *(.data .data.*)     } :program
+	.bss    : { *(.bss .bss.*)       } :program
 
 	firmware_ram_size = . - ram_min;
 

src/asm/ram64.s

@@ -1,4 +1,4 @@
-.section .ram64, "ax"
+.section .text, "ax"
 .global ram64_start
 .code64