Remove references to deprecated external_asm feature

Signed-off-by: Joe Richey <joerichey@google.com>

Author: josephlr

.github/workflows/build.yml

@@ -64,7 +64,7 @@ jobs:
       uses: actions-rs/cargo@v1
       with:
         command: doc
-        args: --no-default-features --features external_asm,instructions
+        args: --no-default-features --features instructions
       if: runner.os != 'Windows'
 
     - name: "Run cargo doc without default features"
@@ -83,14 +83,14 @@ jobs:
       uses: actions-rs/cargo@v1
       with:
         command: build
-        args: --no-default-features --features external_asm,instructions
+        args: --no-default-features --features instructions
       if: runner.os != 'Windows'
 
     - name: "Run cargo build for stable on musl"
       uses: actions-rs/cargo@v1
       with:
         command: build
-        args: --target x86_64-unknown-linux-musl --no-default-features --features external_asm,instructions
+        args: --target x86_64-unknown-linux-musl --no-default-features --features instructions
       if: runner.os == 'Linux'
 
     - name: "Run cargo test"
@@ -102,14 +102,14 @@ jobs:
       uses: actions-rs/cargo@v1
       with:
         command: test
-        args: --no-default-features --features external_asm,instructions
+        args: --no-default-features --features instructions
       if: runner.os != 'Windows'
 
     - name: "Run cargo test for stable on musl"
       uses: actions-rs/cargo@v1
       with:
         command: test
-        args: --target x86_64-unknown-linux-musl --no-default-features --features external_asm,instructions
+        args: --target x86_64-unknown-linux-musl --no-default-features --features instructions
       if: runner.os == 'Linux'
 
     - name: "Install Rustup Targets"

README.md

@@ -9,7 +9,6 @@ Support for x86_64 specific instructions (e.g. TLB flush), registers (e.g. contr
 
 * `nightly`: Enables features only available on nightly Rust; enabled by default.
 * `instructions`: Enabled by default, turns on x86\_64 specific instructions, and dependent features. Only available for x86\_64 targets.
-* `external_asm`: Use this to build with non-nightly rust. Needs `default-features = false, features = ["instructions"]`. Is unsupported on Windows.
 
 ## Building with stable rust